It is well known in the art that optical fibers, e.g. silica fibers with claddings and coatings for optical and mechanical purposes, are relatively fragile and must be protected during manufacture of an optical fiber cable and the installation of such a cable. Thus, the fibers must not be bent below a pre-determined radius and not be subjected to excessive pulling or tensile forces or compression forces. In addition, such fibers are subjected to deterioration in the presence of moisture and must be protected from moisture as well.
In the past, the optical fibers have been loosely enclosed in a plastic tube (buffer tube) having a bore of a cross-sectioned area larger than the cross-sectional area of the fibers or the cross-sectional area of the fibers with the coatings including a plastic which forms a ribbon incorporating a plurality of fibers. Frequently, the axial length of the tube is shorter than the linear length of the fibers or ribbons, and has sufficient tolerance so that stretching or contraction of the tube, which has a relatively high temperature coefficient of expansion and which has a relatively low tensile strength, will not damage the enclosed fibers. Of course, with such construction the tube is able to move axially with respect to the fibers, but the relative axial lengths of the tubes and fibers are selected so that the interior of the tube is not in continuous contact with the fibers or ribbons and so that the fibers are not bent to a too small radius.
For moisture protection, the tube is filled with a water blocking compound of a known type which is sufficiently fluid as not to prevent, significantly, movement of the fibers or ribbons with respect to the tube. Typically, the water blocking compound is a gel or grease-like and non-hygroscopic and/or thixotropic.
While such a tube-optical fiber combination called a "core", provides some protection for the fibers, it usually is not installed in the field for optical communication purposes because of its fragileness and inability to tolerate pulling and bending forces encountered in such installation operations. In further manufacturing steps, additional layers of materials, such as armoring for crushing and rodent protection, strength members to resist pulling and compression forces and a plastic jacket for weather and abrasion purposes are applied to the core. During such further manufacturing steps, the optical fibers must also be protected against damage by mechanical force and moisture.
As strength members, metal wires or high stength non-metallic rods or fibers, such as glass rods or fibers or aramid in matrix of resin or similar materials have been applied to the exterior surface of the core tube in various configurations, for example, helically or longitudinally, with or without circumferential or axial spacing. Such strength members usually have a relatively small cross-sectional area and without lateral support, are easily bent when subjected to compression forces longitudinally thereof. While such strength members provide adequate axial tensile strength, normally, without additional means of support, they provide little resistance to axial compression forces applied to the cable or the core, such as externally thereto or by reason of thermal contraction. It has been proposed that the strength members be made of non-metallic fibers impregnated with an epoxy resin to make the strength members rigid and to resist axial compression forces as well as axial tensile forces. However, such latter strength members make the core and subsequent cable relatively stiff or rigid which is undesirable and they cannot withstand relatively large longitudinal compression forces without undesirable bending, buckling or kinking.
A problem in the design of optical fiber cables is that in many cases, the cables are required to be operable, without damage to the optical fibers, over a temperature range of about -50.degree. C. to about 85.degree. C. although for special situations, the range can be from about -10.degree. to about 65.degree. C. Plastics which are normally used for the cable jacket and the buffer tube have a relatively high temperature coefficient of expansion and contraction and a relatively low tensile strength whereas the optical fibers have a relatively low temperature coefficient of expansion and contraction. In order to prevent the plastic components from applying stress to the optical fibers, the optical fibers are decoupled from the plastic components, such as by making the cross-sectional area of the bore of the buffer tube larger than the cross-sectional area of the optical fibers, by making the linear or longitudinal length of the optical fibers greater than the axial length of the buffer tube and by including in the cable, strength members which have some resistance to contraction of the cable, as aforesaid.
However, it has been found that these measures are not sufficient because depending upon the ratio of the cross-sectional area of the bore of the buffer tube to the cross-sectional area of the optical fibers, the ratio of the excess length of the optical fibers to the length of the buffer tube, the temperature coefficients and thickness of the plastic components, the coupling between the strength members and the plastic components and the tensile strength and compression resistance of the strength members, the optical fibers can still be damaged by reason of stretching and contraction of the plastic components because while the strength members of the prior art which are of small diameter, can, with sufficient coupling to the plastic components, provide adequate protection for the optical fibers with expansion and pulling forces, they cannot provide adequate protection with respect to contraction forces because of the ease with which the strength members can bend or kink with contraction forces. In fact it has been found that with prior art thicknesses of jacket plastic materials,the strength members can pierce the outer jacket with conventional temperature changes thereby rendering the cable unsuitable for further use.
It is, of course, possible to increase the diameters or number of strength members to increase the resistance of the cable to contraction forces, but unless the diameters are increased significantly, thereby significantly increasing the cable diameter, or causing other problems, e.g. manufacturing problems, or resistance to contraction forces cannot be obtained.
When the strength members are merely laid on the surface of the core tube longitudinally or are so laid with a tape or cord merely to hold them in place, the strength members do not provide the desired compression resistance even when the strength members are surrounded by a metal shield or plastic jacket. Thus, when the strength members are subjected to longitudinal compression forces, they slide axially with respect to the core tube, jacket on other components and/or kink or buckle. When this happens, the core tube and hence, the optical fibers are subject to damage. Such kinking or buckling can be sufficient to tear or pierce layers around the strength members.